Scanning method and apparatus comprising a buoyancy material for scanning an underwater pipeline or a process vessel

ABSTRACT

Disclosed herein are a scanning method and apparatus suitable for scanning a pipeline or process vessel in which a beam of gamma radiation from a source is emitted through the vessel to be detected by an array of detectors which are each collimated to detect radiation over a narrow angle relative to the width of the emitted radiation beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 15/653,887, filedJul. 19, 2017 (now U.S. Pat. No. 10,641,716, issued on May 5, 2020),which is a continuation of U.S. Ser. No. 14/355,268, filed Apr. 30, 2014(now U.S. Pat. No. 9,897,558, issued Feb. 20, 2018), which is the U.S.national stage entry of PCT/GB2012/052737, filed Nov. 2, 2012, whichclaims priority to U.S. Provisional App. Nos. 61/597,272, 61/597,237,and 61/597,354, each filed on Feb. 10, 2012, and which also claimspriority to UK Application No. 1200744.9, filed Jan. 17, 2012, UKApplication No. 1118944.6, filed Nov. 2, 2011, and UK Application No.1118943.8, filed Nov. 2, 2011, the entire disclosures of each of whichare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method of scanning a structure todetect changes in density by means of detecting radiation emitted by aradiation source by a radiation detector.

DESCRIPTION OF THE RELATED ART

Methods of imaging objects and animals by X-ray tomography andpositron-emission tomography are well-known, particularly in the fieldof medical imaging for diagnostic purposes. U.S. Pat. No. 4,338,521describes an X-ray scanner for use in computerised tomography which hasa detector comprising an array of detector modules, comprising aplurality of photodiodes and a plurality of scintillator crystals and aradiation beam collimator to direct collimated radiation to thescintillator crystals. A fan-shaped beam of x-rays from an x-ray sourceis directed through a patient to be detected by the detector. The sourceand detector are rotated around the patient to provide the data fromwhich a tomographic image may be constructed. In positron-emissiontomography (PET) a positron emitted by the decay of a radionuclideannihilates on contact with a suitable electron, causing the emission oftwo gamma photons of 511 keV in opposite directions. The detection ofthe direction of the gamma photons enables the estimation of thelocation of the annihilation event and thus the radionuclide within thepatient. The PET scanner therefore incorporates an array of detectorswhich can detect gamma photons placed around the body of a patient. Animage of the relative concentration of the radionuclide in the body maybe constructed from the number of photons detected at each detector.

Whilst these scanning methods are well-developed and have become commonfor medical scanning, scanning a dense structure such as a pipelinepresents difficulties because the density of the pipeline material issuch that radiographic scanning must be done using gamma radiation whichis of sufficient energy to penetrate and pass through the structure sothat at least some radiation can be detected after the beam has passedthrough the structure. Gamma scanning of structures such as distillationcolumns is a standard industrial diagnostic method for measuring changesin density at different parts of the structure, for example to determinethe location and integrity of column trays or other internal structureswithin the column. Normally this type of scanning is carried out with asingle gamma source placed adjacent the column to emit a beam ofradiation through the column and a radiation detector placed on anopposed part of the column to intersect and measure the radiation thathas traversed the column between the source and detector. The source anddetector are normally moved so that different parts of the column can bescanned. The use of many different positions and more than one source ordetector can provide sufficient data for generating density maps, ortomographs, of the structure which is scanned, although the resolutionis generally quite coarse. In order to generate higher resolutiondensity maps, information from a much larger number of radiation pathsthrough the structure must be used than is currently achieved withconventional column scanning methods.

The inspection of pipelines to find flaws such as wall loss, cracks orcorrosion pitting is an application in which it would be desirable touse radiation scanning. A known problem for the oil and gas productionindustry is the inspection of pipelines located underwater, inparticular on the sea-bed.

Inspection of the interior of the pipeline by the use of pigs is notalways possible, for example when the pipeline has varying diameter.Inspection from outside the pipe may be carried out by ultrasonicmethods, although this is not suitable for pipelines having aninsulation or coating. Gamma scanning can produce useful informationabout the density through a cross-section of the pipe. In order toproduce information about the thickness of the walls of the pipe atsufficiently high resolution to identify small flaws that may be presentin the walls of the pipe a large number of radiation paths through thepipe need to be scanned. Furthermore, if a fan-shaped radiation beam isto be used to scan the pipe, many of the radiation paths pass through achord of the pipe cross-section and therefore through a relatively largeamount of the pipeline wall material, requiring a relatively high energygamma source. In order to detect the gamma radiation that has passedthrough the structure it is necessary to use detectors of sufficientsize and density to stop the gamma photons so they do not pass throughthe detector undetected. In order to maintain a high resolution thecollimation of the detectors has to be sufficient to significantlyreduce detection of gamma photons which have been scattered from a pathother than the direct path to a particular detector. The detectors needto be small enough to provide good spatial resolution. A large number ofdetectors is needed to achieve a reasonable measurement time. The use ofheavy collimation on a large number of detectors necessitates a scanningapparatus which is very heavy and so rotation of the apparatus in acontrolled and precise manner around a large pipeline becomes verydifficult. When the pipeline is horizontal, it becomes necessary totrench the pipe in order to allow sufficient space in which to move ascanning apparatus and so the use of large apparatus becomes expensive,particularly when the pipeline is located sub-sea. All of theseconsiderations bring particular problems to the application of highresolution x-ray tomography methods to the scanning of pipelines orother large structures with high energy gamma radiation.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide such a method,although the method of the invention may be useful for scanningstructures other than pipelines and for use in locations including bothdry and sub-sea locations.

The present invention relates to a method of scanning a structure todetect its physical properties. In particular the invention relates to amethod of scanning an elongate structure, such as a pipeline, to detectchanges in its material density which may indicate variations in wallthickness caused by corrosion or erosion or to deduce information aboutthe contents of the pipeline such as the build-up of deposits or thenature of fluid flowing within the pipeline. Typically, the method andapparatus concerns the measurement of density of a structure by means ofdetecting radiation emitted by a radiation source by a radiationdetector.

According to the invention, we provide a method of scanning a structureto detect changes in density between different parts of the structurecomprising the steps of:

-   -   a) providing at least one source of gamma radiation, and a        plurality of detector units capable of detecting said gamma        radiation,    -   each said detector unit comprising:        -   i. a radiation detector comprising a scintillator comprising            a scintillating material and having a detecting surface            defined by its thickness t and height h, wherein t≤h at the            detecting surface and having a depth d perpendicular to the            detecting surface at least 2t, and        -   ii. a photodetector for detecting light emitted by the            scintillator in response to gamma radiation, and        -   iii. a collimator placed between the scintillator and the            source of radiation;    -   b) causing said source to emit gamma radiation along a        predetermined radiation path towards said detector, wherein said        path passes through at least a portion of said structure;    -   c) measuring the number of photons of gamma radiation detected        by each one of said detectors;    -   d) calculating a density value for each path from the        measurement of photons detected by the detector associated with        the respective path.

According to the invention, we provide an apparatus for scanning astructure to detect changes in density between different parts of thestructure comprising:

-   -   at least one source unit comprising a source of gamma radiation        and shielding material arranged to restrict the emission of        gamma radiation from the source unit: a plurality of detector        units capable of detecting said gamma radiation, each said        detector unit comprising:        -   i. a radiation detector comprising a scintillator comprising            a scintillating material and having a detecting surface            defined by its thickness t and height h, wherein t≤h at the            detecting surface and having a depth d perpendicular to the            detecting surface at least 2t, and        -   ii. a photodetector for detecting light emitted by the            scintillator in response to gamma radiation, and        -   iii. a collimator placed between the scintillator and the            source of radiation;    -   and data processing means for calculating a density value for        each path from the measurement of photons detected by the        detector associated with the respective path.

The apparatus of the invention is suitable for use in the scanningmethod of the invention, in which a target structure is scanned todetect changes in its shape or composition by means of passing radiationemitted by a radiation source through the structure and detectingradiation after it has passed through the structure. The method works onthe well-known principle that the amount of radiation attenuated orscattered by an object is related to the mass of material the radiationhas passed through. By measuring the amount of radiation detectedthrough each selected path through the target structure it is possibleto calculate and/or compare the density of the structure along oneradiation path with the density of the structure along a differentradiation path. By “density value” we mean a value which represents theactual or relative density of the structure which lies on a particularpath from the source to a particular detector. The density value maytake the form of a number of counts of gamma photons or a normalised,smoothed or comparative number of counts of gamma photons. Alternativelythe density value may be a value calculated from the number of counts ofgamma photons. The density value may be expressed graphically, includingas an image or part thereof. The relative dimensions of thescintillator(s) of the radiation detector of the invention allow aplurality of scintillators to be placed in close proximity in order toachieve a high degree of spacial resolution of detected radiation sothat characteristics of small portions of the structure may be detectedwith high precision. The method is particularly useful for scanning aregular structure such as a pipe, although the method and apparatus maybe used for scanning other types of structure. In a particularembodiment of the invention, the scanning method is a method ofdetecting changes in the density of the wall of a pipeline. Use of thismethod enables flaws such as voids, cracks, scale, gas hydrates orthinning to be detected. The change in density may be detected relativeto adjacent portions of the pipe wall or relative to a reference valuegenerated from a model pipeline or a calculated value.

In the method of the invention, an array of detector units is mountedopposite at least one source of gamma radiation such that the radiationis emitted in the direction of the detecting surfaces. The targetstructure to be scanned is capable of being interposed between thesource and detector unit so that the radiation emitted by the source canpass along a plurality of paths through a portion of the structure andimpinge upon the detecting surfaces. The source and detector unit may bemoved relative to the target structure (or vice versa) in order to scandifferent portions of the structure. The principal benefit of using anarray of detectors is that different paths through the structure may bescanned simultaneously. Each of the paths has the shape of a frustum,having the source at the apex and the detecting surface of a detector atthe base. Each detector in the array defines a different path throughthe structure so that the number of paths which may be scannedsimultaneously is equal to the number of detectors in the array. Thenumber of detectors in a detector array may vary from fewer than 10 tomore than 100, e.g. up to 150, depending on the application for whichthe scanning method is to be used. In practice, the mass of shieldingmaterial required to shield and collimate a large number of detectorsmay provide a practical upper limit to the number that can be used.

The source unit and detector unit may be mounted on a support in fixedrelationship to one another or the detector unit may be movable relativeto the source unit. It is greatly preferred that the source unit anddetector unit are mounted in a fixed relationship when the apparatus isin operation. This enables the apparatus of the invention to provide aprecise and fixed alignment of source and detector units so thatmodulation of the counts measured by the detectors can be attributedsolely to the materials between the source and detector through whichthe radiation path passes. In this way, very small differences in thedensity of such materials can be detected, allowing the detection ofsmall flaws or changes in the thickness of a pipeline wall. The sourceand detector unit are preferably mounted so that the detectingsurface(s) of each of the detectors form a tangent to an arc having thesource at its origin. The plurality of detector units are arranged inclose proximity to each other. It is preferred that the array ofdetector units is arranged in the form of an arc having a radius centredon the centre of the object or structure to be scanned. The design ofthe detector unit preferably minimises the total distance on eachdetection path through each detector unit in order to make the array ofdetector units as compact as possible, whilst maintaining sufficientdepth of the collimation and a detector for efficient detection of gammaphotons on each path.

In a preferred form of the invention, the source unit and detector unitsare mounted on a support which provides means for the structure to bescanned, or a portion thereof, to be located between the source unit andthe detector unit. The support maintains the source unit and detectorunit in a spaced apart fixed relationship. The support thereforeincludes means for mounting at least one source unit and means formounting a plurality of detector units on the support. The support maycomprise an elongate portion or “arm” having first and second opposedends to which said detector unit and support unit may be mounted orjoined. The means for mounting a detector unit comprises a detectorhousing joined to the support. The support, source unit, and/or detectorhousing may be formed as a unitary component or from separate componentswhich are joined together. The support must be sufficiently strong towithstand supporting and moving the detector and source units withoutdeformation and sufficiently rigid to maintain a precisely fixedrelationship between the source unit and detector housing, including anydetector units housed therein. One suitable material for the supportcomprises an aluminium alloy, which may be machined by known methods toform the required shape for the support.

The detector housing is shaped to house one or more detector units andto secure such units so that they do not move unintentionally, duringoperation of the apparatus. It is an important feature of the preferredapparatus that the detector units can be maintained in a fixedrelationship to the source during use in a scanning method. The detectorhousing may be of such size and shape to house several detector units atthe same time, for example from 2-100 units. The detector housing mayinclude means to house a detector unit in more than one position withinthe housing. The means may simply comprise a detector housing havingsufficient space to house a detector unit in more than one locationwithin the housing. Means, such as guide rails or a motor may also beincluded to move one or more detector units from a first location withinthe housing to a second location within the housing. A practical limiton the resolution of a scanning method using an array of detectors isthat the spacing between each one must be sufficient to allow a minimumrequired amount of shielding to ensure that each detector is adequatelyshielded from photons impinging on a neighbouring detector. Even whenhighly dense alloys are used for detector shielding, we have found thata practical limitation on detector spacing is approximately 1° of arc.In one embodiment of the apparatus, the detector housing is of such asize as to allow a detector unit to be housed in at least two positions,offset from each other by a distance which is a fraction of the distancebetween the detectors. When the fraction is 0.5 of the detector spacingdistance (0.5 s), the resolution of the apparatus may be doubled bycarrying out a first scan when the detector array is in a first positionin the housing and then repeating the scan when the detector array is ina second position in the housing which is offset from the first positionby 0.5 s. If additional positions are provided, and or, the angulardistance between them is reduced, additional scans can provideadditional data to enhance the resolution of the scan. The detector maybe moved between any of the at least two positions, for example byoperation of a powered switch operated by a solenoid. The provision ofmeans to lock the detector array securely in a single position whistscanning is greatly preferred. Such means may comprise a sprung pin orboss engaging with an indexing hole in each of the desired positions.

In a particularly preferred form, the apparatus comprising support,detector housing (including any detector unit therein) and source unitmay be moved laterally and/or rotationally, relative to the structureand means are provided to effect such movement. Preferably the sourceunit and detector units are rotated around the structure such that theradius of rotation has an origin within the structure, for example theorigin may be approximately at the geometric centre of the structure inthe plane of rotation. The means for said movement may include motorisedor manual impulsion and guiding means such as rails, tracks, guidechannels or locating indicators, to guide the path of rotation.Preferably the apparatus is provided with at least one track or rail,shaped to conform to at least a part of the structure to be scanned. Forpipeline scanning, for example, one or more arcuate tracks may beprovided so that the apparatus may be moved along the tracks, forexample by means of a worm drive or a stepper motor turning a splineddrive-wheel, gear or cog to rotate the detector housing and sourcearound the circumference of the structure. In a preferred form, theguiding means is indexed, for example by providing indentations intowhich teeth of a drive cog may engage to effect movement of the scanningapparatus. The provision of indexed movement may provide a predeterminednumber of scanning locations at known angular positions around thestructure. Preferably a means is provided to rotate the detector unitsand source around a circumference of the structure to be scanned. In thecase of a cylindrical object, such as a pipeline, the detector units andsource are rotated around the circumference of the pipeline. Thescanning method is carried out at a plurality of radially offsetpositions around the structure so that density data may be acquired at avariety of angles through the structure.

The guiding means, e.g. tracks, may extend partially or entirely aroundthe pipeline. It is preferred to move the source and detector unitcontinuously around the structure in order to avoid the problems, suchas damage to the apparatus or slippage of the scanning system,associated with successively accelerating and braking the apparatus.More than one scan may be required to gather sufficient data todetermine the structure properties, although the number of scans and thescanning time is dependent upon the density and mass of material throughwhich the radiation must travel from the source to the detector units.Preferably a continuous rotational movement around the structure at arelatively low rpm, for example at from about 1 to about 20 rpm,especially from 1-10 rpm is maintained during the scanning operation.Therefore in a preferred apparatus means such as a continuous track areprovided to enable such movement. The guiding means may be provide inmore than one part, which, following deployment of the apparatus, arebrought together and optionally joined, to form the desired length oftrack for scanning. The source unit, detector unit, support and guidingmeans may all be housed within an enclosure which is capable ofsurrounding at least a part of the structure. The enclosure may have anopen position in which it may be positioned around the structure and aclosed position in which it is capable of scanning the structure. Theenclosure may take the form of a hinged pair or set of jaws which may beclamped to the structure to be scanned.

Power may be recovered from the movement of the apparatus by means of adynamo, or similar, which may be then used to help power the detectorsor other operating systems of the apparatus.

Movement of the apparatus may also involve lifting and/or sliding theapparatus manually or by mechanical means, for example by means of aremotely operated vehicle (ROV). An ROV may be preferred to deploy andmove the apparatus when deployed in remote or underwater locations.Linear movement, for example parallel to the axis of a pipeline or avessel, may be achieved by means of a crawler mechanism or using a trackor rails, or alternatively by external means such as a lifting apparatusor ROV. The movement means may include indexing, for example at aparticular angular separation in order to provide a predetermined numberof scanning locations at known positions around the structure. For anapplication such as scanning a pipeline, the movement may be controlledby means of a programmed electronic control unit, for example to executea predetermined timed movement or set of movements of the source anddetector units relative to the pipeline. The movement may be rotationalto scan around the circumference of the pipeline and/or lateral to moveaxially along the pipeline.

The apparatus may comprise means to support the apparatus in proximityto the structure to be scanned. Such means may comprise clamps, whichare capable of engaging the structure and supporting the scanningapparatus in one or more positions on the structure. The clamps may bemanually operated but mechanically operated clamps are preferred.

The scintillating material is selected according to the properties ofthe radiation which is to be detected and the conditions in which thedetector is deployed. In principle, any suitable scintillating materialmay be selected and many materials are known and marketed for thedetection of radiation. A high density material provides a greatercapacity to stop radiation in a given volume and consequently thescintillator can be made smaller than would be possible for a lowerdensity material. A small scintillator is more stable, for example it isless likely to exhibit a temperature differential between differentparts of the crystal. Smaller crystals transmit light more effectivelyrequiring lower-powered photo-detectors to be used. Of importance forthe present application a small scintillator can have a small detectingsurface and so radiation travelling along a narrow path can be detectedwithout a significant amount of incident radiation from background orscattered radiation from the same or different paths. For the detectionof gamma radiation, it is preferred to use a dense inorganic material sothat the incident photons may be stopped using as small a detector aspossible. Scintillating materials having a density >5 and a highZ-number (atomic number) are preferred. It is preferred that thescintillator(s) have a depth and density that enable them to stop >80%of gamma photons of energy of 662 keV. For use in applications requiringresistance to environmental conditions, especially moisture, anon-hygroscopic crystal scintillator should be selected. Especiallypreferred detectors for use with gamma radiation include BGO (bismuthgermanate), CdWO₄, LaBr₃(Ce), LYSO (lutetium yttriumoxyorthosilicate—cerium doped), LSO (lutetium oxyorthosilicate—ceriumdoped) and CeF₃ (cerium fluoride). When a mechanically rugged detectoris required, a crystal having no cleavage planes may be preferred inorder to increase its resistance to shattering following a thermal ormechanical shock.

Each scintillator has a detecting surface, which, in use is arranged tointersect the radiation path so that the radiation impinges upon thedetecting surface. Other surfaces of the detector which are not arrangedto be detecting surfaces will be referred to as non-detecting surfaces.Although any part of a scintillator is normally capable of detectingphotons, the designation in this specification of detecting surfaces andnon-detecting surfaces refers to the arrangement of the scintillator inthe detector unit for detecting radiation from a source. Thescintillator also has a surface through which light generated by thescintillator in response to photons impinging upon the detecting surfaceleaves the scintillator. This surface is referred to herein as thecollecting surface. The collecting surface is arranged in opticalcommunication with the photodetector. The collecting surface can be incontact with the photodetector or it may be separated therefrom by oneor more light transmitters, in the form of a window, lens, opticalfibre, light pipe or optically coupling material etc. made from amaterial which transmits the light generated by the scintillator to thephotodetector. The collecting surface of the detector may have a similarcross-sectional area and shape to that of the photodetector window or itmay be different. The detector itself may act as a light guide to pass asubstantial proportion of the light generated in the scintillator to thephotodetector. In this context, the use of the phrase “substantialproportion” means all of the light generated in the scintillator ispassed to the photodetector, save for a proportion of light that isunintentionally lost due to the efficiency of the light transmissionbeing less than 100%.

Each of the detectors comprises a scintillator, normally supported in asuitable position so that the detecting surface intersects a path ofradiation emitted by the source at a particular distance from and aparticular angle to the radiation source. It is a particular feature ofthe invention that the detector can substantially reduce the detectionof scattered radiation and increase the precision with which radiationemitted by a source along a particular linear path is detected. Theprovision of a detector having an elongate shape in which t≤h, morepreferably <0.5 h, at the detecting surface enables the detectors to belocated in close proximity so that spatial resolution of each detectoris high. The depth of the detector contributes to the stoppingefficiency of the detector so that a detector having a depth dperpendicular to the detecting surface at least 2t, more preferably atleast 5t, especially >10t is preferred in order to stop and measureenergetic photons.

The smallest dimension of the detecting surface of the scintillator ispreferably between about 1 mm and about 10 mm. The smallest dimension isdefined to be the thickness t of the material. More preferably, 1 mm≤t≤5mm and in a preferred embodiment t is about 5 mm. Preferably thedetecting surface is generally rectangular so that the area of thesurface is defined as t×h, where h is in the range 5-100 mm. Morepreferably, 10 mm≤h≤50 mm and in a preferred embodiment h is about 25-40mm. The depth, d of the scintillator is in the range 10-100 mm. Morepreferably, 25 mm≤d≤75 mm and in a preferred embodiment d is about 40-60mm.

A material which is impermeable to the radiation may cover a part of thedetecting surface of the scintillator to delimit the portion of thedetecting surface on which radiation may impinge. The collimator mayoverlap and cover one or more edges of crystal by up to about 5 mm.

The detection of scattered photons is preferably further reduced bypreventing the detectable radiation from impinging upon the surfaces ofthe detector which are not detecting surfaces. This may typically beachieved by covering the non-detecting surfaces, except for the portionof the collecting surface in optical communication with thephotodetector, with a material which prevents transmission of theradiation to the non-detecting surfaces. In a preferred embodiment thedetectors are surrounded by shielding material so that all of thenon-detecting surfaces, except for the portion of the collecting surfacein optical communication with the photodetector, are protected fromradiation. By shielding material we mean a material which is highlyattenuating to the radiation which is to be detected by the detector.Typically, a shielding material for protection from ionising radiationsuch as gamma radiation includes lead and heavy metal alloys. Suchmaterials are well known to persons skilled in the art of designingradiation detectors and nucleonic instruments.

When the scintillator is thin, scintillation light generated as a resultof the interaction of a gamma photon with the scintillation material islikely to be internally reflected several times before it enters thephotodetector. Since each reflection may be less than 100% efficient,the capacity for multiple reflections provides multiple opportunitiesfor loss of light and thus a decrease in the detection efficiency of thedetector. It is therefore preferred to provide the non-detectingsurfaces with means to reflect light internally within the detector.Preferably the non-detecting surfaces are coated with a super-reflectivecoating, capable of reflecting at least 95% of the light within thescintillator and more preferably at least 98% of that light.

When the detector unit comprises more than one detector, deployed in theform of an array of detectors, a preferred embodiment of the inventioncomprises a block of shielding material (a “detector block”) havingopenings extending inwardly from a surface of the block, each openingcontaining a detector, the detecting surface being accessible toradiation from outside the block. A portion of the detecting surface maybe covered by shielding material for the purposes of delimiting the areaof the detecting surface or for mechanically retaining the detectorwithin the opening. The non-detecting surfaces of the detector mayoptionally be enclosed partially or wholly within the opening andcovered by the shielding material. The detector block includes means bywhich the collecting surface of the scintillator(s) may be brought intocontact with a photodetector or a light transmitter. Such means may takethe form of an open passage through which the scintillator extends sothat the collecting surface is accessible to the photodetector or lighttransmitter.

The precision of the detector is increased by providing collimationmeans for restricting the path along which radiation may travel to thedetecting surface. The collimation means comprises a collimator formedfrom a shielding material and arranged so that radiation travellingtowards the detecting surface from selected directions may contact thedetecting surface whilst radiation travelling from non-selecteddirections is excluded from the detecting surface. In this way, onlyradiation travelling along selected paths from a radiation source to thedetector may be detected. The collimation may be arranged so thatradiation from one or more selected radiation sources is detected.Suitable design of the collimation can significantly reduce thedetection of scattered photons, which are usually deflected from thepath along which they were emitted by the source. Alternatively, thecollimation may be designed so that scattered photons and othersecondary radiation is detected preferentially. In a preferredembodiment the collimation means comprises a block of shielding materialhaving a channel, or preferably a plurality of channels extendingthrough. The collimator block comprises a plurality of channels, eachchannel being formed through the block and corresponding in position toone of the detectors in said array. Each channel is shaped to define thepath of radiation which is to be detected by each scintillator. Eachchannel has an opening at the end proximal to the scintillator which ispreferably mounted over the detecting surface of the scintillator sothat the detecting surface, or a portion of it, is within the opening ofthe channel. The end of a channel distal to the scintillator is open toallow radiation to enter the channel and travel to the scintillator. Theopening preferably lies on the plane of a tangent to a circle having thesource as its origin. The area of the distal opening defines the maximumuseful area through which radiation can pass to the detecting surface.The channel walls are normally straight. The length of the channel(s) isdetermined according to the requirements of the detector and the energyof radiation emitted by the source. A longer channel reduces thedetection of scattered or reflected radiation more than a shorterchannel and so the resolution of detection of a particular path ofradiation is higher. The length of the collimation channels may bedetermined by the skilled person according to the type of radiationwhich is to be collimated, in accordance with known principles ofphysics. Generally for collimating radiation from a caesium source(which is a preferred source for use in the method and apparatus of theinvention), a collimation depth of at least 50 mm should be used. Acobalt source requires more collimation and generally a depth of atleast 75-80 mm would be used. Americium emits less energetic gammaradiation and requires only about 20 mm of collimation depth. Americiummay be used in some applications but would not be suitable for use inscanning steel pipelines, which is a preferred application. The depth, dof the collimator channels is preferably in the range from 30-150 mm.More preferably 50≤d≤150 and, for use with a caesium source suitable forscanning large pipelines d is most preferably about 80-120 mm.

The cross-section of the channel may be any convenient shape, althoughit is preferred that the channel has the same shape and orientation asthe detecting surface. Often, the channel has a generally rectangularcross section. The shape and/or size of the channel cross-section maychange along the length of the channel, or they may remain substantiallyconstant. In a preferred embodiment, at least one of the collimatorchannels has at least one wall defining the channel which is alignedwith a radius of a circle having the source as its origin. Preferablyeach of the walls of the channel is aligned with a different radius ofsaid circle so that the opening of the channel is aligned to facedirectly towards the source. Preferably, in such an arrangement the endof the collimator distal to the scintillator has an opening lying on theplane of a tangent to a circle having the source as its origin. In thisway the detection of photons travelling in a straight line from thesource, through the target structure along the collimator channel to thedetector may be maximised for any given area of detecting surface. Morepreferably all of the collimator channels have at least one wall andpreferably all of their walls, aligned with the radius of a circlehaving the source as its origin. In such an arrangement the walls of thecollimator channels are not parallel to each other and all of thechannels face the direction of the source. When this alignment of thecollimator channels is adopted, and the array of detector units isarranged in an arc having an origin which is not the source, at leastsome the collimator channels do not extend in a direction which isperpendicular to a tangent to that arc. This is a preferred arrangementfor scanning a cylindrical structure such as a pipeline. In order toproduce the collimator channels having this preferred alignment, it ispreferred to form each channel in a block of shielding material by meansof a machining method. For this reason the use of plates of shieldingmaterial, e.g. steel plates, of the type found in the detector units ofx-ray tomography apparatus (for example as described in U.S. Pat. No.4,338,521) is not preferred.

In one embodiment of the apparatus of the invention, the detector unitcomprises a collimator block and a detector block, joined together suchthat the proximal end of each channel is in register with the detectingsurface of a detector. The detector block and collimator block arejoined together so that the connection between them does not allowradiation to impinge on the detecting surface of a detector which hasnot travelled through a channel in register with the detecting surface.It may be possible to form the detector block and the collimator blocfrom a single piece of shielding material but it is normally easier tomanufacture them separately and then join them together.

The collimator block may be formed from a dense shielding material suchas lead or a heavy alloy which attenuates gamma radiation.Alternatively, the collimator block may be formed, at least in part,from a less dense material, such as steel for example, which providesless shielding but which is not as heavy as the more dense shieldingmaterials such as lead or heavy alloy. In one version of such acollimator, collimator channels are formed from a first material, suchas steel, and a layer of a second material, such as a heavy alloy,having a greater shielding capacity than the first material, ispositioned over at least one external surface of the detector unit. Inthis way the detector unit may be better protected from the impact ofscattered radiation from selected directions than from other directions.In practice, it is possible to determine, by calculation and/ormodelling, the probability at which gamma radiation scattered fromparticular angles will impinge upon the detector unit. This informationmay then be used to provide more shielding over those surfaces of thedetector unit at which scattered gamma photons are more likely tocontact the detector units. More shielding can be provided either byusing a more dense material or by increasing the thickness of theshielding material. One advantage of providing different shielding atdifferent parts of the detector unit, or forming the detector unit fromdifferent materials is that the weight of the detector unit can bereduced whilst the shielding and collimation of the detectors issubstantially maintained. A further advantage of using a material suchas a steel to form at least a part of the collimator is gained if thematerial has greater structural strength than a traditional denseshielding material such as lead or heavy alloy so that less structuralsupport must be used to support the collimator block.

The photodetector may be a photodiode, photomultiplier tube (PMT) orother suitable light detecting device. Currently, PMTs are preferred tophotodiodes because they are more sensitive to very low levels of light,although the use of other photodetectors, such as siliconphotomultipliers or avalanche photodiodes may become preferable astechnology develops. The photodetector generates an electrical signal inresponse to light entering it through an optical window. The wavelengthsdetected by the photodetector should be matched as far as possible tothe wavelengths generated by the scintillator to maximise the detectionefficiency. Normally a photodetector is provided for each scintillatorso that the amount of radiation detected by each scintillator can bemeasured independently of the other scintillators.

The photodetectors are held in position by attachment means such as aclamp or mounting. When more than one photodetector is present, they maybe mounted in fixed positions within a mounting block. The mountingblock is formed from a material which is impermeable to light and to anyother radiation which is likely to affect the signal produced by thephotodetector. The photodetector is mounted with its optical windowoptically coupled to a collecting surface of the scintillator. Thephotodetector may be coupled using an optically coupling adhesive.Selection of a suitable optical coupling material such as an adhesivehaving some resilient elastic properties can provide the detector unitwith some resistance to the effects of vibration or impact shock.Normally the photodetector is adjacent the scintillator, but it may bephysically separated from the scintillator if light transmitting meansare provided to transmit light from the scintillator to thephotodetector. In that case it is important that the efficiency of thelight transmission is as high as possible.

The photodetector may be in a coaxial relationship with its respectivescintillator and collimator. Alternatively, the photodetector may bemounted at an angle to the axis of the collimator and scintillator, forexample at an angle of between about 45 and 100 degrees to that axis,especially about 90°. One advantage of mounting the photodetector at anangle to the axis of the scintillator and collimator is that the totaldepth of the detector unit may be reduced compared with a detector unitin which the photodetector is mounted coaxially. Reducing the depth ofthe detector unit helps to minimise the space needed around the targetstructure to carry out a scan and this can allow scanning in restrictedspaces and/or minimise the need for trenching a pipeline prior toscanning.

In a preferred embodiment of the invention an array of n detector unitsis provided, comprising an array of n radiation detector comprising: nscintillators,

n photodetectors, each photodetector being optically coupled with arespective scintillator,

a detector block made of highly attenuating material incorporating aplurality of n channels extending through the detector block from afirst surface to a second surface, each channel being sized toaccommodate a single scintillator, and

a collimator block comprising a block of shielding material having nchannels extending therethrough, and wherein the collimator block isjoined to the detector block so that each channel is in register with ascintillator.

wherein each scintillator is located within a channel in the detectorblock, where n=an integer in the range from 2-150.

Each detecting surface preferably forms a tangent to an arc of a circlehaving a radiation source as its origin. In one embodiment, eachdetector surface forms a tangent to the surface of a part of a spherehaving the radiation source as its origin.

The source unit comprises a source of penetrating radiation, asource-holder and a collimator. The collimator and source-holder may becombined. The collimator is formed of a material which is highlyattenuating to the radiation emitted by the source and is normallyformed of a heavy alloy material of the type known and commonly used forshielding radiation of the appropriate energy and type. The collimatoris located and adapted to limit the radiation emitted by the source unitto a predetermined beam shape and direction. Preferably the radiationbeam is shaped by the collimator to form a fan, cone or frusto-cone, orsector in each case having the source as origin. A preferred beam shapeis a cylindrical sector, i.e. a sector having a thickness rather thanbeing planar. Preferably the beam is collimated to provide a beam areaat the location of the detector(s) which has the same general shape andarea as the combined detecting surface(s) of the array of detectors. Inthe preferred form of the apparatus, the source unit is mounted on asupport, preferably in the region of an end of an elongate support.

The radiation source is selected by the transparency to the radiation ofthe material(s) to be measured, e.g. a vessel and/or its contents (i.e.the attenuation coefficient of the medium) and the availability ofsuitable sources and detectors. For scanning large solid structures suchas process vessels and pipelines, suitable sources of gamma include ⁶⁰Coand ¹³⁷Cs ¹³³Ba, ²⁴¹Am, ²⁴Na and ¹⁸²Ta, however any gamma-emittingisotope of sufficient penetrating power could be used, and many such arealready routinely used in density gauges, such as those used as levelmeasurement devices.

Usually, the half-life of the radioisotope used will be at least 2, anddesirably at least 10, years. The half-lives of the radioisotopesmentioned above are: ¹³⁷Cs gamma about 30 years, ¹³³Ba about 10 yearsand ²⁴¹Am about 430 years. Suitable sources generally emit radiation atenergies between about 40 and 1500 keV.

The source unit may include one or more than one source. The scanningmethod may utilise more than one source unit if required.

The apparatus further comprises a signal/data processor for operating onthe electrical signal from the detectors in the detector unit(s) and acontroller to control the operation of the apparatus. Signalsrepresentative of the counts of photons detected by the scintillatorsare processed by the data processor. The signal may be subjected tosmoothing or stabilisation algorithms, averaged or otherwise operated onaccording to standard practices. A data processor may performcalculations based on the signal from the radiation detector or from asignal processor if present. The data processor may output informationconcerning the amount of radiation measured over a time interval, or itmay further calculate derived properties of the scanned structure,usually in the form of a bulk density or a change in bulk densitybetween radiation paths through the structure. The scanning method iscarried out at a plurality of radially offset positions around thestructure so that density data may be acquired at a variety of anglesthrough the structure and a tomography algorithm may be used to provideinformation about the changes in density at different paths through thestructure. In a preferred form the data from the detectors is operatedon by the data processing unit using tomography algorithms in order toproduce a graphical representation of the density or composition of thestructure along different paths. The data processor may contain acalibration or information concerning the radiation source. The dataprocessor output is may be connected a display or a (optionallywireless) transmission means so that a signal can be sent from theapparatus to a remote location. Alternatively a signal comprising datafrom the radiation detector itself may be sent, for processing at aremote location. A power supply is provided to power the photodetectors,data processor and control electronics and also to power motors formoving the apparatus.

In use in the scanning method of the invention, the apparatus isdeployed so that the source unit and detector units are positioned inrelation to the structure to be scanned so that one or more radiationpaths from the source to detectors in the detector unit pass through thedesired portion of the structure. The amount of radiation, in the formof counts, is measured by the detector in each detector unit deployed inthe apparatus. The scanning method is carried out at a plurality ofradially offset positions around the structure so that density data maybe acquired at a variety of angles through the structure. The apparatusmay then be moved to a different location or orientation with respect tothe structure and the measurement is repeated. In this way a record ofthe attenuation to radiation through each radiation path through thestructure may be gathered and used to calculate the location of changesor to build a representation of the structure and its contents.Information such as changes in density which may highlight flaws orother features within the structure can be obtained from the datagathered from the detectors using data analysis tools known for use intomography methods.

For operation underwater, it is preferred to increase the buoyancy ofthe apparatus by means of a buoyancy block. If used, the buoyancy blockmay be attached to the apparatus by means of a flexible attachment sothat the flotation force produced can be balanced during movement of theapparatus. Additionally or as an alternative, spaces within theapparatus may contain a foam material in order to provide positivebuoyancy to the apparatus. Parts of the apparatus may be coated in aresilient foam material, again for the purpose of providing buoyancy andalso to protect the apparatus from physical damage, such as impactdamage, and environmentally induced damage such as corrosion.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described with reference to the attacheddrawings, which are:

FIG. 1A: a schematic view of a scintillator suitable for use in thescanning method and apparatus of the invention

FIG. 1B: A view of the scintillator of FIG. 1A from direction A.

FIG. 1C: A schematic view of an alternative scintillator suitable foruse in the scanning method and apparatus of the invention.

FIG. 2: a schematic view through a section of a detector unit.

FIG. 3: a schematic view through a longitudinal section of a detectorunit

FIG. 4: a schematic view of a detector block forming part of a radiationdetector according to the invention.

FIG. 5: a schematic view of a photomultiplier mounting block formingpart of a radiation detector according to the invention.

FIG. 6: a schematic view of a collimator block forming part of aradiation detector according to the invention.

FIG. 7: a schematic view through a section of an alternative detectorunit

FIG. 8: a view of a front elevation of the detector unit of FIG. 7.

FIG. 9: a diagrammatic view of a preferred arrangement of the apparatus.

FIG. 10: a schematic view of an array of detector units for an apparatusaccording to the invention.

FIG. 11: a schematic view of a part of an apparatus according to theinvention.

FIG. 12: a schematic elevation view of an apparatus according to theinvention.

FIG. 13: a schematic perspective view of the apparatus shown in FIG. 12.

FIG. 14: a schematic elevation view of an apparatus according to theinvention, and

FIG. 15: a schematic elevation view of an apparatus according to theinvention.

FIGS. 1A and 1B show a bismuth germanate (BGO) scintillator crystal 10having a thickness t of 5 mm, a height h of 30 mm and a depth d of 75mm. The detecting surface 12 is opposite the collecting surface 14. Allsurfaces of the bismuth germanate (BGO) scintillator crystal 10 exceptthe detecting surface 12 and collecting surface 14 are coated in ahighly reflective coating. FIG. 1C shows an alternative bismuthgermanate (BGO) scintillator crystal 10.

FIG. 2 shows, a transverse cross-section through a detector unit 30,comprising a block of heavy alloy 16 which is highly attenuating toradiation, of the type used as shielding material for gamma radiation.The block of heavy alloy 16 has collimation channels 18 extending fromthe front face to the opposed rear face of the block of heavy alloy 16.In use, a bismuth germanate (BGO) scintillator crystal 10 is housed theblock of heavy alloy 16, with the detecting surface 12 of the bismuthgermanate (BGO) scintillator crystal 10 at the front face of the blockof heavy alloy 16 and the collecting surface 14 optically connected tophotomultiplier tube (PMT) 20. Photomultiplier tube (PMT) 20A isconnected to an adjacent bismuth germanate (BGO) scintillator crystal 10(not shown), and is shown to demonstrate the packing of thephotomultiplier tubes (PMTs) within the detector unit 30. The detectorunit 30 comprises nineteen detectors, each comprising a bismuthgermanate (BGO) scintillator crystal 10 and a photomultiplier tube (PMT)20 and mounted in register with a collimation channel 18 in the block ofheavy alloy 16.

FIG. 3 shows a longitudinal section through an assembled radiationdetector comprising a collimator block 40, a detector block 200 and aPMT mounting block 300, each shown individually in FIGS. 4-6. The blocksare mounted together so that channels 46, 36 and 26 are all in register,together forming channels extending from the front face of thecollimator block 40 to the rear of the PMT mounting block 300. A bismuthgermanate (BGO) scintillator crystal 10 is housed within channel 26 anda PMT 50 is housed within channel 36. The PMT 50 may be connected toelectronic data processing and control apparatus by means of connectorsaccessible from the rear of the channel 36. Channels 46 a, 36 a and 26 aand the bismuth germanate (BGO) scintillator crystal 10a and PMT 50a areshown in dashed outline because they are not in the same plane as therespective channels and components in solid outline. In the embodimentshown, the longitudinal axes of the channels 26, 36, and 46 form anangle of between 1 and 2° with the longitudinal axis 51 of each block200, 300, and 40.

FIG. 4 shows a detector block 200 which comprises a rectangular block ofheavy alloy 21 which is highly attenuating to radiation, of the typeused as shielding material for gamma radiation. The rectangular block ofheavy alloy 21 has channels 26 extending from the front face 22 of therectangular block of heavy alloy 21 to the opposed rear face 24. Blindsockets 28 are provided for locating and mounting a collimator block 40.The channels 26 are sized to house a BGO scintillator crystal 10. Inuse, a BGO scintillator crystal 10 is housed in each channel 26, withthe detecting surface 12 of the BGO scintillator crystal 10 at the frontface 22 of the rectangular block of heavy alloy 21 and the collectingsurface 14 at the rear face 24.

FIG. 5 shows a photomultiplier mounting block 300 which comprises arectangular block of white plastic material such aspolytetrafluoroethane. Channels 36 extend from the front face 32 of thephotomultiplier mounting block 300 to the opposed rear face 34. Thechannels 36 are each of an appropriate size to house a smallphotomultiplier tube 50. The channels 36 are positioned in thephotomultiplier mounting block 300 in such a way that each opening atthe front face 32 of the photomultiplier mounting block 300 buttsagainst the collecting surface 14 of a BGO scintillator crystal 10mounted in an adjacent detector block 200 when the rear face 24 of thedetector block 200 is placed against the front face 32 of the PMTmounting block 300. Blind sockets 38 are provided for locating andmounting to the detector block 200.

FIG. 6 shows a collimator block 40 comprising a rectangular block ofheavy alloy which is highly attenuating to radiation, of the type usedas shielding material for gamma radiation. The collimator block 40 haschannels 46 extending from the front face 42 of the collimator block 40to the opposed rear face 44. Blind sockets 48 are provided for locatingand mounting a detector block 200. The channels 46 have a width andheight which is slightly less than the width and height of the channels26 in the detector block 200. In an assembled radiation detector, rearface 44 of collimator block 40 is mounted against the front face 22 ofdetector block 200 such that channels 46 are in register with channels26.

In FIG. 10, a detector unit 30 consisting of 19 collimation channels 18,BGO scintillator crystals 10 and photomultiplier tubes (PMTs) 20 isshown, in which the collimation channels 18 are formed into a singleblock being spaced apart and angled from each other by an angle of about1 degree of arc.

FIGS. 7 and 8 show an alternative arrangement for a detector unit. InFIG. 7, the steel block 60 forming the collimator 62 and holding thescintillator 64 and PMT 66 is formed from a stainless steel. The PMT 66is mounted out of alignment with the radiation direction in order toreduce the total depth of the detector unit 30. The direction ofradiation is indicated by the arrow. FIG. 8 shows an elevation from thedirection of the arrow. Layers 68 and 69 of a dense heavy alloyshielding material are positioned above and below the steel block 60.This material provides additional shielding for the detectors fromscattered radiation impinging on the detector unit 30.

FIG. 11 shows a support 70 joined rigidly at one end to a generallyarcuate shaped detector housing 72, all formed of an aluminium alloy,and at the other end to a source unit 74. An arrangement of the sourceunit 74 and detector units around steel pipe 82 is shown in FIG. 9. Thedirection of three collimator channels 46 a, 46 b, and 46 c isillustrated in order to show that they align with the direction of thesource unit 74 and are not aligned with the radius Rt of the structure.The source unit 74 and detector housing 72 is arranged to rotate about acentral point on the structure on a path having a radius Rt. The sourceunit 74 comprises a cesium source 78 of gamma radiation surrounded byheavy alloy shielding material 76 including a slot for collimatingradiation in a fan shaped beam 80 towards the detector housing 72. Thedetector housing 72 comprises an aluminium alloy cage and, in theembodiment shown, contains two arcuate arrays of detector units 30, oneat each end. The detector housing 72 includes rails along which thedetector units 30 may be moved to different locations within thedetector housing 72. The detector housing 72 shown could accommodate oneor more additional detector units 30 if required.

FIGS. 12 and 13 show an apparatus for scanning steel pipe 82, having aninternal diameter of about 234 mm and a wall thickness of about 43 mm,to detect changes and flaws in the wall. The pipe wall is surrounded bya layer of insulating material 84. A support member 86 is damped to thepipeline by means of clamps 88 hydraulically operated by arms 90. Asupport member 86 is clamped to the pipeline by means of clamps 88hydraulically operated by anus 90. The apparatus may include a buoyancyblock comprising a buoyancy material 87 that is attached to theapparatus by means of a flexible attachment 89 (as in FIG. 12), or mayinclude a space-filling foam or a coating, comprising a buoyancymaterial 87 on the apparatus (as in FIG. 13), in order to providepositive buoyancy. The support member 86 also supports rails 92 whichsupport the support 70, detector housing 72 and source unit 74. A motor94 mounted on the detector housing 72 is operable to move the detectorhousing 72 and source unit 74 along the rails 92 and thereby to rotatethe position of the source unit 74 and detector units 30 around thepipeline. At each position, radiation emitted by the source unit 74towards each detector unit 30 in the two detector units forms a numberof radiation paths through the pipe wall and insulation equal to thenumber of detector units 30, which in this case is (19×2)=38 separatepaths which can be scanned at the same time. When the apparatus isrotated to a different position along the rails 92, a further 38 pathscan be scanned. Data in the form of counts detected by the detectorunits 30 is processed and stored by a data processor housed in housing96 located towards the top of the support 70. Deployment of the detectorunits 30 in the positions shown is particularly suitable for scanningthe pipeline walls and insulation to detect flaws and changes betweendifferent locations in the pipe 82.

In the detector housing 72 shown, there is space for one or moredetector units 30 to be placed in the central portion of the housing 96.In that position, a detector unit 30 would detect radiation which haspassed through the lumen of the pipe 82 and its contents. Use of adetector unit 30 in such a position would therefore be suitable forconducting tomography scans of the pipe 82 and contents.

FIGS. 14-15 show another embodiment of a scanning apparatus according tothe invention. The apparatus comprises two parts of a hinged housing 102which together form a clamp with jaws which can be opened (FIG. 14) andclosed (FIG. 15) around the pipe 82 by operation of a hydraulic cylinder98. When closed, the hinged housings 102 surround the pipe 82 but arespaced apart from the surface of the pipe 82. Rollers 100 contact thesurface of the pipe 82 and maintain the spacing of the hinged housing102 from the pipe 82. Hinged housing 102 covers and contains a detectorhousing for one or more arrays of detector units 30 and a source unit 74as described above. The source unit 30 and detector unit 30 are mountedin a fixed relationship to one another and are arranged to move along atrack within the jaws so as to rotate around the circumference of thepipeline. Adjustable grippers 104 are present either side of the pipe 82which are operable by means of a hydraulic cylinder 108 to grip the pipe82 and centralise it within the space between the jaws and the pipe 82.When the hinged housing 102 is closed around the pipe 82 and centralisedby the adjustable grippers 104, the source unit 74 and detector housing72 are rotated around the pipe 82 so that density information can beacquired by the detector units 30 at a plurality of angular locationsaround the pipe 82. The data is then processed to produce a tomographyimage or an indication of one or more properties of the pipeline atdifferent locations around the path of the scanning operation. Whensufficient data has been acquired, the hinged housing 102 is opened andmoved to a different location along the pipeline for new scanning datato be acquired.

The invention claimed is:
 1. A method of inspecting an underwaterpipeline to determine wall thickness or information about contents ofthe underwater pipeline, the method comprising: providing a gammaradiation source and an array of detector units on an apparatus thatcomprises buoyancy material; interposing the underwater pipeline betweenthe gamma radiation source and the array of detector units so thatradiation emitted by the gamma radiation source passes along a pluralityof paths through a portion of the underwater pipeline and impinges uponthe array of detector units; acquiring data at a plurality of radiallyoffset positions around the underwater pipeline to acquire density dataat a variety of angles through the underwater pipeline; and presenting arepresentation of the underwater pipeline or contents of the underwaterpipeline using the density data.
 2. The method according to claim 1,further comprising: rotating at least one of the gamma radiation sourceand the array of detector units around a circumference of the underwaterpipeline when acquiring data.
 3. The method according to claim 1,wherein the apparatus is hinged so that the apparatus is configured tobe opened and closed around the underwater pipeline.
 4. The methodaccording to claim 1, wherein the representation is a representation ofa composition or contents of the underwater pipeline.
 5. The methodaccording to claim 1, wherein presenting a representation of theunderwater pipeline comprises building the representation usingtomography algorithms.
 6. The method according to claim 1, furthercomprising detecting a void in the underwater pipeline.
 7. The methodaccording to claim 1, further comprising detecting a crack in theunderwater pipeline.
 8. The method according to claim 1, furthercomprising detecting wall thinning in the underwater pipeline.
 9. Themethod according to claim 1, further comprising detecting a gas hydratewithin the underwater pipeline.
 10. The method according to claim 1,further comprising detecting scale within the underwater pipeline. 11.The method according to claim 1, further comprising detecting a changein a density relative to a reference value.
 12. The method according toclaim 11, wherein the reference value is a calculated value.
 13. Themethod according to claim 11, wherein the reference value is a valuefrom an adjacent portion of the underwater pipeline.